How to call select_linked_pick from script?
Managed to solve this issue by simply using different operators to get the same result:

    loc = event.mouse_region_x, event.mouse_region_y

    bpy.ops.view3d.select(location=loc)
    bpy.ops.mesh.select_linked()


Found from this answer by CoDEmanX.

Seemed a lot simpler in the end than trying to retrieve the right indexes for everything to use select_linked_pick...